The present invention relates to servo systems for use with tape media and, in particular, to a servo system that makes use of optical tracking features formed on the back side of the tape to enable the system to precisely align the read/write heads with the recorded data located on the front side of the tape.
It is a problem in the field of tape data storage media to accurately position the read/write heads with respect to the tracks of data written on the tape. In longitudinal tape media, the tape typically contains a single recording surface on which is written along the length of the tape a plurality of parallel aligned tracks of data. In systems using high track densities ( greater than 200 tracks per inch), the precise alignment of these data tracks with the read/write heads of the associated tape drive is accomplished by the use of a plurality of servo tracks written on the tape recording surface, interspersed with the tracks of data. The servo tracks, typically written during the tape manufacturing process, function as a physical reference for the placement of the data tracks when the data tracks are written on the tape and for reading previously written data tracks from the tape. The position information derived from the servo tracks is used by the tape drive to adjust the position of the movable read/write head to enable the accurate reading and writing of data to and from the data tracks.
A problem with this tracking system is that the number of data tracks written on the tape is limited by the need for servo tracks written on the recording surface to provide position information. There is a need to facilitate the development and use of future tape systems with increased data capacity. This is often accomplished by the increase in the number of data tracks and the amount of data placed in a given track. Due to the mechanical instability of tape media, higher data track densities require a decreased physical spacing between data and servo tracks to ensure the accurate alignment of the read/write heads with the data tracks. The decreased spacing drives the need for increased numbers of servo tracks which, in turn, must share the tape area with data tracks. Furthermore, it is highly desirable for future systems to retain the ability to read the tapes made on earlier systemsxe2x80x94this is termed xe2x80x9cbackward compatibilityxe2x80x9d. This backward compatibility requires the head positioning servos to be able to work on tapes with varying numbers of tracks and track configurations. This presents a design challenge and can force future drive designs to trade off between performance enhancements and backward compatibility. A further problem is that magnetically written servo tracks are susceptible to track erasure. Bulk erasure of the tape can erase servo tracks, drive system failure can result in the servo tracks being overwritten and corruptedxe2x80x94either of which can render the tape and its data useless.
There are numerous servo track systems in use in the field of rewritable data storage media. Some of these are illustrated by the disclosures of the following patents.
U.S. Pat. No. 4,958,245, titled xe2x80x9cApparatus And Method For Optical Servo Control With Media Having Information Storage And Servo Control Regions Of Different Reflectivitiesxe2x80x9d discloses an optical servo head to read position information from a disk on which data is magnetically recorded. The disk has a plurality of optical servo tracks formed thereon in the form of relatively nonreflective regions comprising concentric grooves formed in the reflective surface of the magnetic disk. The servo system illuminates a plurality of the reflective and non-reflective regions and uses a quadrature photodetector array to achieve tracking.
U.S. Pat. No. 5,067,039 titled xe2x80x9cHigh Track Density Magnetic Media With Pitted Optical Servo Tracks And Method For Stamping The Tracks On The Mediaxe2x80x9d discloses a method for mechanically stamping the servo tracks on the optical disk during the disk manufacturing process.
U.S. Pat. No. 5,279,775 titled xe2x80x9cAcousto-Optic Intensity Control Of Laser Beam During Etching Of Optical Servo Information Of Magnetic Mediaxe2x80x9d discloses a system that etches servo tracks on a magnetic disk. Track following during the etching process is accomplished by the use of an acoustic-optical device to maintain the beam in concentric patterns, while a laser beam is used to etch the servo tracks, with the laser beam intensity being controlled by the acoustic-optical device.
U.S. Pat. No. 5,283,773 titled xe2x80x9cSteering Laser Beam While Etching Optical Servo Tracks For Magnetic Disksxe2x80x9d discloses a system that etches servo tracks on a magnetic disk. Track following during the etching process is accomplished by the use of an acoustic-optical device to maintain the beam in concentric patterns, while a laser beam is used to etch the servo tracks.
U.S. Pat. No. 5,462,823, titled xe2x80x9cMagnetic Recording Materials Provided With A Photosensitive Layerxe2x80x9d discloses a magnetic recording element that comprises a support layer coated with a magnetic recording layer and a photosensitive layer. Optical tracking information is formed on the photosensitive layer by the exposure of the photoreactive surface using a servo track mask.
The above noted servo systems all make use of servo tracks that are formed on the rewritable media on the same surface as is used to store the data. The servo information is typically in the form of servo tracks that are formed coextensive with the data tracks and interspersed among the data tracks. Therefore, the servo tracks occupy space on the tape that can be used for the storage of data. Furthermore, there is an inherent interaction between the use of servo tracks and the writing of data tracks such that the system cannot optimize the data recording function without impacting on the servo function. Conversely, the system can not optimize the servo function without impacting the data recording function.
The above described problems are solved and a technical advance achieved by the present optical servo system for a tape drive that functions to align a read/write head with the data tracks written on a recording surface of a tape by reading optical servo tracks that are formed on the back side of the tape. This process decouples the magnetic recording of data on the recording surface of the tape from the optical servo system which makes use of servo tracks formed on the back side of the tape to position the read/write head. For example, the recording formats of the data can be altered and the number of data tracks can be changed without impacting the optical servo system. The servo system can accommodate a wide range of recording format changes within its signal processing algorithms without modifying its servo tracks. In addition, the data storage capacity of the tape is increased since the entire recording surface of the tape is filled with data tracks and precise alignment of the read/write head with the data tracks makes it possible to place the data tracks closer together.
The tape used in this system has magnetic data tracks recorded on the front side of the tape and-optical servo tracks, comprising regions of differing reflectivity or phase, formed on the back side of the tape. Although current magnetic media types could be utilized, the servo track reading and writing processes explained below are optimized by the use of a media with a second side optically tuned to have high contrast or phase change at the read lumination wavelength and high write sensitivity at the servo track writing wavelength. A magnetic read/write head that is positioned juxtaposed to the front side of the tape reads data from and writes data to the data tracks while the optical servo system reads servo data from the servo tracks that are formed on the back side of the tape. The requirement for close data track-to-servo track spacing is met by having the servo tracks located immediately behind the data tracks. Regions of contrasting reflectivity or phase are also provided on a surface of the read/write head to enable the optical servo system to view an image of both the read/write head and the entire back side of the tape to thereby align the movable read/write head with the data tracks. The head""s optical features may be formed by numerous means known to those familiar with the art including integration into the head structure itself or affixing a secondary structure to a head surface. An optical sensor array generates electrical signals indicative of the received image which are then used by a digital signal processor to determine the required alignment of the read/write head with the data tracks. Once the proper alignment is determined, the digital signal processor generates a position error signal that in turn is fed to the servo amplifier which drives an actuator to align the movable read/write head with the data tracks.
The use of the two sources of optical data from the read/write head and the tape media improves the accuracy, performance and reliability of the data track to read/write head alignment while simplifying the entire servo system. Using this approach, all optical components can be fixed in place. Since the servo system xe2x80x9ccloses the loopxe2x80x9d around the tape and head optical feature alignment, system alignment and calibration requirements are eased. Fault tolerance to damaged tape and/or head optical features is facilitated by the availability of redundant optical informationxe2x80x94a plurality of optical features exist on both the tape and the head. Furthermore, because this system locates the entire width of the tape with respect to the head, servo information is always available to quickly re-establish head-to-tape alignment (i.e. xe2x80x9ctrack followingxe2x80x9d) should it be lost during drive operations. Interchange, the ability to read a given tape on a population of tape drives, is facilitated by the servo system""s ability to image the mechanical relationship between tape and head immediately after the tape is loaded into the drive and make appropriate offset adjustments in the head""s static position. The immediate availability of this offset information results in a reduced tape load time. This reduction, in turn, results in higher overall job throughput for the drive in a repetitive tape loading environmentxe2x80x94such as is commonly seen when robots are used to mount and dismount tapes in the drive.
The optical servo tracks are not subject to magnetic erasure. Accidental magnetic damage is eliminated thus increasing data recovery reliability. Bulk magnetic erasure of the tapes facilitates their reuse by reducing old data noise sources, improving data security by eliminating old data and providing an economic benefit over new tapes.